With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data transmission performance over the copper wire local loop transmission lines that connect telephone central offices (COs) to customer premises (CPs). In conventional telephony networks, customer premises equipment (CPE) are coupled to CO switches over the above mentioned transmission lines, which are commonly known as “local loops,” “subscriber lines,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. In the art, the term “line” and “loop” are used interchangeably, both terms referring to the copper wire pair used in a typical telephone transmission line conductor. Historically, the public switched telephone network (PSTN) evolved with subscriber loops coupled to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. “Central office” or “CO” means any site where a subscriber loop couples to a telephony switching unit, such as a public switched telephone network (PSTN), a private branch exchange (PBX) telephony system, or any other location functionally coupling subscriber loops to a telephony network. Digital service provision to the CP is a more recent development. With it, the telephone network has evolved from a system capable of only carrying analog voice communications into a system that can simultaneously carry voice and digital data.
Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost migration to digital technologies. Subscriber loops capable of carrying digital signals are known as digital subscriber lines (DSLs). Various digital technologies provide customers with additional flexibility and enhanced services by utilizing frequency-division multiplexing (FDM) and/or echo-canceling (EC) and/or time-division multiplexing (TDM) techniques to fully exploit the transmission capability of a subscriber loop. These newer DSL technologies provide digital service to the customer premises without significantly interfering with the existing plain old telephone service (POTS) equipment and wiring by utilizing portions of the available frequency spectrum not used by a POTS signal. These portions of the frequency spectrum are often referred to as “logical channels.” Logical channels within a subscriber line that carry digital signals are known as “DSL channels,” while logical channels within a subscriber line which carry POTS analog signals are known as “POTS channels.”
DSL technologies, such as but not limited to integrated services digital network (ISDN), high-bit-rate digital subscriber line (HDSL), HDSL2 and symmetric digital subscriber line (SDSL), utilize echo-canceled pulse amplitude modulation to create a baseband data transmission spectrum and therefore do not coexist with a POTS signal which typically utilizes the 0-4 kilo-hertz (KHz) portion of the available frequency spectrum.
Other DSL technologies coexist with POTS by frequency-division multiplexing (FDM) a single data signal onto a logical channel above (at higher frequencies than) the 0 KHz to 4 KHz frequency range used by the analog POTS signals. Such multiplexing techniques and terminology are common to those skilled in the art, and are not described in detail herein. Examples of DSL technologies compatible with POTS include, but are not limited to, Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), Very High Speed DSL (VDSL), Multiple Virtual Lines (MVL™) and Tripleplay™. Communications systems employing DSL-over-POTS technology may frequency multiplex a plurality of data signals and a single POTS signal onto a single subscriber line. ADSL system employing time-division multiplexing would multiplex a plurality of data signals onto a single logical channel with each different data signal allocated to a predefined portion of time in a predefined, repeating time period. Note that an advantage of TDM is that the transmitter does not actively transmit at all times.
FIG. 1 is a simplified illustrative block diagram of a portion of an existing telephony system 20. A telephone company central office (CO) 22 coupled to communication system network 24 via connection 26. Residing in the CO 22 is at least a signal front end system and a plurality of multiple transceiver units 30 and 32. Each multiple transceiver unit has a plurality of transceiver port cards. For illustration purposes, the multiple transceiver unit 30 has three transceiver port cards 34, 36 and 38. Multiple transceiver unit 32 has two transceiver port cards 40 and 42. A transceiver port card typically includes at least a transmitter signal generating circuitry unit (not shown) for decoding and encoding communication signals into proper formats, and a transmitter (not shown) and a receiver (not shown).
Signal front end system 28 detects incoming communication signals from network 24 which are to be transmitted to any one of a plurality of customer premises (CP). Signal front end system 28 performs the necessary signal processing of the communication signal received from network 24, via connection 26, and passes the communication signal to each of the transceiver port cards 34, 36, 38, 40 and 42, via connection 46. For convenience of illustration, signal front end system 28 is shown connected to the transceiver port cards 34, 36, 38, 40 and 42 via a single connection 46. Alternatively, signal front end system 28 may be connected to the transceiver port cards 34, 36, 38, 40 and 42 via a plurality of individual connections or in any other convenient manner. Similarly, connection 26 is shown as a single connection for convenience.
The transmitter signal generation circuitry (not shown) further process the communication signal. A common example of the transmitter signal generation circuitry known to those skilled in the art is a digital signal processor (DSP). Such processing may include modulation of the communication signal for transmission to a CP 44, and demodulation of a communication signal received from CP 44. The transmitter signal generation circuitry passes the processed communication signal to a transmitter (not shown) residing in the transceiver port card 34, 36, 38, 40 and 42. The transmitter then provides the necessary communication signal amplification so that a communication signal having the proper signal strength can be transmitted, via the output connection 48 of the transceiver port card 34, 36, 38, 40 and 42, onto a subscriber loop 50, for transmission to a CP 44. For convenience of illustration, three CP 44s are shown. Typically, CO 22 would be in communication with hundreds of CPs.
Subscriber loop 50 may be any suitable connection for communicating electrical signals, but is typically a copper wire pair, as is well known in the art, that was originally designed to carry a 0-4 KHz analog voice channel (POTS signal). When a copper wire pair is used for data signal transmission, the wire pair is often referred to as a digital subscriber loop (DSL).
Many other components typically reside in CO 22 which are not illustrated in FIG. 1 for convenience. For example, no digital receiver circuitry, POTS signal circuitry, couplers between the POTS and the digital systems, power supplies and line splitters are shown in FIG. 1. Such components are not described in detail herein as these components are well known in the art. Furthermore, not all of the components residing in the signal front end system 28 or the transceiver port card 34, 36, 38, 40 and 42 are described herein in detail or illustrated in FIGS. 1-3 other than to the extent necessary.
Located within the CP 44 may be a plurality of digital equipment devices which transmit and receive data signals over subscriber loop 50. For convenience of illustration, a personal computer (PC) 52 is shown residing in CP 44 and coupled to subscriber loop 50. Illustrative examples of other digital equipment devices include, but are not limited to, facsimile (FAX) machines, set top boxes, internet appliances, computers or the like. PC 52 includes a modem (not shown), or the like, coupled to subscriber loop 50. PC 52 may communicate with a plurality of other digital equipment devices (not shown) via an Ethernet (not shown), other local access network (LAN), or the like (not shown). PC 52 includes user interface devices, such as keyboard 54 and/or viewing screen 56, to interface with a user (not shown).
A modem (not shown), typically residing in PC 52, decodes a data signal received from the transmitter (not shown) over the subscriber loop 50. The modem also transmits communication signals onto the subscriber loop 50 which have been generated by PC 52 or other similarly functioning digital device residing in the CP 44, to a receiver (not shown) residing in the transceiver port card 34, 36, 38, 40 and 42. Typically data is communicated using a communication signal that has been modulated. Modulation schemes used to communicate between CO 22 and a CP 44 may include, but are not limited to, carrierless amplitude/phase modulation (CAP), quadrature amplitude modulation (QAM), Discrete Multi Tone (DMT) or pulse amplitude modulation (PAM), and are commonly known in the art and are not described in detail herein.
Prior art digital communication systems, like the signal front end system 28 and the multiple transceiver unit 30 having a plurality of transceiver port cards 34, 36, 38, 40 and 42, illustrated in FIG. 1, are often added into an existing CO 22 so that the digital communication system can utilize existing POTS facilities, such as subscriber loops, power supplies, building structures, grounding and protection facilities, etc. Also, it may be desirable to expand already existing digital communication facilities residing in the CO 22. However, electrical code requirements, regulations and/or rules pertaining to the heat generated by communication system components may limit the size of the digital communication system addition or expansion. Such code requirements specify the maximum heat generation allowed per unit size of floor space and/or per unit size of cabinet volume.
Each transceiver port card 34, 36, 38, 40 and 42 in the rack has a specified heat dissipation value based upon maximum theoretical utilization of the transceiver and its associated components (the transceiver residing in the transceiver port card running at a full data transmission or data receive rate). Thus, the maximum number of transceiver port cards 34, 36, 38, 40 and 42 that may be installed into a single multiple transceiver unit 30 is determined by summing each individual port card theoretical maximum heat dissipation value up to the total maximum heat dissipation allowed in a single multiple transceiver unit 30, as specified by code. Because of the large amount of heat dissipated by the components on a single transceiver port card 34, 36, 38, 40 and 42 when running at its maximum theoretical utilization rate, the number of transceiver port cards 34, 36, 38, 40 and 42 that may be installed in a single multiple transceiver unit 30 is limited in prior art communication systems.
Due to the statistical nature of digital data communications, which are typically bursty in nature (brief periods of heavy data transmission followed by periods of inactivity), transceiver port cards 34, 36, 38, 40 and 42 are rarely each running at their maximum theoretical utilization rate. Furthermore, the probability that all transceiver port cards 34, 36, 38, 40 and 42 will be operating at their maximum theoretical utilization rate at the same time is very low. Therefore, it can be expected with reasonable certainty that the amount of heat dissipation for the multiple transceiver unit will not reach the maximum heat dissipation specified by code in a typical installation. Thus, fewer transceiver port cards are installed in a multiple transceiver unit than the number of transceiver port cards that could be installed if a more realistic transceiver port card heat dissipation rate could be realized.
Consequently, a way to guarantee that a single transceiver port card 34, 36, 38, 40 and 42 will operate at a specified heat dissipation rate less than the maximum theoretical utilization rate is desirable. Such a guarantee would allow code certification of a single multiple transceiver unit 30 having a greater number of transceiver port cards 34, 36, 38, 40 and 42, thereby utilizing previously unused space typically available in the multiple transceiver unit 30. Therefore, it is desirable to limit power consumption in transceiver port cards 34, 36, 38, 40 and 42 in an efficient and cost effective manner. Reducing power consumption in the transceiver port cards 34, 36, 38, 40 and 42 would facilitate a more compact construction of an electrical code compliant digital communication system.